Early Roman Calendars

Much of the knowledge we now have about early Roman calendars came from Ovid, a Roman born in 43 B.C., and from Plutarch, a Greek biographer who wrote between A.D. 105 and 115.1 Both of them had access to historical documents that are no longer extant. Ovid claimed that his information was “dug up in archaic calendars,” 2 so it was already ancient over two thousand years ago.
We can assume that Rome’s original citizens brought from their birthplace the notion of calendars having ten major divisions because early-on theirs contained only ten months. It has been suggested that those month lengths reflected growth cycles of crops and cattle. When compared with the solar year, it had an uncounted winter period of approximately sixty days.

Plutarch pointed out that months at the time of Rome’s founding were of varying lengths, some as short as twenty days and others with thirty-five or more in what early Romans believed was a year of three hundred and sixty days. 3 Read more about his comments at Plutarch on the Early Roman Calendar.
Romulus, the legendary first Roman king, was said to have made extensive changes to those month lengths, assigning twenty-nine days to some and thirty-one to others. If you are interested in legends, you might want to read Romulus in Mythology.
Both Ovid and Plutarch made it clear that Martius, originally the first month, was named after Mars, the Roman god of war. Six of the other original ten were simply numbered as Quintilis thru Decembris (fifth thru tenth) but there were already disagreements when Ovid wrote, two thousand years ago, as to the sources of names for what were originally the second thru fourth, Aprilis, Maius and Junius. These disagreements continue to the present time.
When writing about April, Ovid said “I have come to the fourth month, full of honor for you; Venus, you know both the poet and the month are yours.” 4 Someone later pointed out that “April was sacred to Venus, and her festival–the Festum Veneris and Fortuna Virilis–occurred on the first day of this month.” Apparently Aprilis stems from aphrilis, corrupted from Aphrodite, a Greek name for Venus. Jakob Grimm, a later authority, opposed this stating it may have originated from the name of a god or hero named Aper or Aprus.” 5
Maius was said by some to be named after the goddess Maia, a daughter of Atlas, and Junius “is indirectly named after the goddess Juno, the Roman equivalent of Frigga.” 6 But Ovid suggested that names of months we now call May and June possibly refer not to sky-gods but rather to elders and young men. 7
The original fifth month, Quintilis, received the name of Julius (July) to honor Julius Caesar. And the original sixth, Sextilis, was named Augustus in honor of the second Caesar. The original seventh thru tenth months retained their names (Septembris, Octobris, Novembris and Decembris) but thru time became the nineth thru twelfth months.
There was disagreement even in Ovid’s day as to the sequence and time at which Januarius and Februarius were added to the original ten months.
Januarius probably became part of the calendar about half a century after the time Rome was founded because Plutarch said that Numa, the king who followed Romulus, made it the first month of the year and made February the last. One historian assigns that action an exact date by stating that “January and February were added to an original Roman calendar of only ten months in 713 B.C.” 8
January was named after Janus, a sky-god who was ancient even at the time of Rome’s founding. Ovid quoted Janus as saying “The ancients called me chaos, for a being from of old am I.” After describing the world’s creation, he again quoted Janus: “It was then that I, till that time a mere ball, a shapeless lump, assumed the face and members of a god.” Numa honored Janus by founding a temple for him. 9 That may have been the reason Plutarch assumed Numa added January to the calendar.
Some say Februarius got its name from a goatskin thong called a februa (“means of purification.”) On the 15th day of this month Romans observed the festival of Lupercalia. During the festival, a februa was wielded by priests who used it to beat women in the belief that it would make a barren woman fertile. However, there’s a Latin verb februare, meaning to “expiate” or “purify.” It seems more reasonable to assume the purification people had in mind when naming the month was that of the calendar year’s length, not that of women upon whom the thong was applied.
Apparently Februarius, when adopted, had but 23 days—traditionally the 23rd day of that month was the end of the calendar year.10 That indicates Februarius may have been observed in pre-Romulan times when months had as few as twenty days. Also, adding five days at year-end (to extend February’s length to 28) is similar to the change made by many other peoples who, around the time of Rome’s founding, added five days to their own calendar, but considered them to be unlucky and not part of the normal year.11

Romans always reconciled differences between calendar and solar year lengths during that “Month of Purification.” Whenever and however Roman calendars were modified to correspond to year length, it was always done after the 23rd day of February, traditionally the last day of the year. Even in our time, leap year is observed with a 29-day February. To purists, “leap day” is February 24, not the 29th.
Plutarch wrote: “Numa…added an intercalary month, to follow February, consisting of twenty-two days, and called by the Romans the month Mercedinus. This amendment, however, itself, in course of time, came to need other amendments.” 12 (When observed, that leap month always immediately followed February 23.)
Here’s what the historian Livy said about Numa’s contribution to the Roman Calendar:

“First of all he divided the year into twelve months, corresponding to the moon’s revolutions. But as the moon does not complete thirty days in each month, and so there are fewer days in the lunar year than in that measured by the course of the sun, he interpolated intercalary months and so arranged them that every twentieth year the days should coincide with the same position of the sun as when they started, the whole twenty years being thus complete. He also established a distinction between the days on which legal business could be transacted and those on which it could not, because it would sometimes be advisable that there should be no business transacted with the people.” 13
Others claim that it wasn’t until 452 B.C. that a month named Intercalaris was added to the Roman calendar in order to add those days required to bring calendar length back into phase with the solar year. This month also began after the 23rd day of Februarius. It was observed every second year and was said to have had a length of either 22 or 23 days, with the remaining five days of Februarius added after them. 14

Shown on the right is a Roman Calendar wall painting for the months of Januarius, Februarius and Martius. It was created at a time January and February were the first two months of the year. Each day in their eight-day marketing week is identified by one of the letters A thru H listed vertically on the far left side of each month’s column.
On the top line between day letter and abbreviation for month name is a “k•,” short for kalendae, the name Romans gave the first day of each month. You can see that the first day of January was identified with the letter A. Special days were indicated by red letters including an A for the first day of each eight day period or an N or other letter in addition to the day letter for some other days. This may well be the source of our own term “Red Letter Days.”
Note that the last day of January is labeled E and the first day of February continues the weekly cycle with an F. Similarly, the last day of February is an A and the first day of March is labeled B.
Roman Numerals for 29, 28 and 31 at the bottom of the painting indicate the number of days in those first three months of the Roman Republican Calendar.
In 45 B.C., Romans modified their method of marking time to keep it in phase with seasons, but not require intercalation of an extra month. They accomplished this with the Julian Calendar. Month lengths were extended to bring the calendar’s total to 365 days, making it truly solar. This change was accompanied by addition of an extra day every fourth year (after February 23rd) because of the almost six extra hours beyond 365 days in a tropical year.
For additiional information about the early Roman calendar see Bill Thayer´s Calendarium.